Shelf Life of Tropical Canarium Nut Stored Under Ambient Conditions
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horticulturae Article Shelf Life of Tropical Canarium Nut Stored under Ambient Conditions David A. Walton 1,*, Bruce W. Randall 1, Matthew Poienou 2, Tio Nevenimo 2, John Moxon 3 and Helen M. Wallace 1 1 Genecology Research Centre, Faculty of Science, Health, Education and Engineering, University of the Sunshine Coast, Maroochydore DC, QLD 4558, Australia; [email protected] (B.W.R.); [email protected] (H.M.W.) 2 National Agriculture Research Institute, Islands Regional Centre, P.O. Box 204, Kokopo 613, Papua New Guinea; [email protected] (M.P.); [email protected] (T.N.) 3 Cocoa Board of Papua New Guinea, P.O. Box 532, Kokopo 613, Papua New Guinea; [email protected] * Correspondence: [email protected]; Tel.: +61-7-5459-4519 Academic Editor: Douglas D. Archbold Received: 24 October 2016; Accepted: 20 January 2017; Published: 3 February 2017 Abstract: There is a need to develop alternative crops to improve the food security and prosperity of developing countries. The tropical nut Canarium indicum (canarium nut) is increasingly used as a shade tree for cocoa and has potential for commercialization as a sustainable crop that will improve food security and livelihoods in Melanesia and East Asia. There is no information on canarium nut shelf life characteristics. Canarium kernels may be prone to rancidity, due to a high content of unsaturated fatty acids. Kernels at 5.4% moisture content were vacuum-packed with a domestic vacuum-packaging system and stored for six months in Papua New Guinea and for nine months in Southeast Queensland, Australia at both ambient temperatures (22 to 31 ◦C and 22 to 25 ◦C, respectively) and under refrigeration. Nuts were analysed for changes in peroxide values and free fatty acids (FFAs) over the storage periods that might indicate development of rancidity. Peroxide values indicated very low levels of oxidation in all treatments. Free fatty acids were at low levels but increased significantly during storage at ambient temperatures. The results suggested that vacuum-packed Canarium nuts can be stored safely under ambient tropical conditions for six months with daytime temperatures around 31 ◦C, and for nine months at 25 ◦C. Increasing FFA levels at ambient temperatures indicate caution about longer storage time at ambient temperatures. Storage under refrigeration greatly prolonged shelf life. Keywords: nuts; Canarium indicum; canarium nut; shelf life; peroxide values; free fatty acids; food security 1. Introduction Only 103 domesticated species of the world’s 250,000 species of flowering plants provide 90% of the world’s food [1]. A diversification away from over-reliance on staple food crops will be important as part of the process of achieving security of food production for an increasing world population in the future [2,3]. There is potential for expansion of species currently used as indigenous food crops. Nuts in particular are increasing in importance as food crops, with total world tree nut production for 2014–2015 at ~3.5 million metric tons (kernels) [4]. Edible nuts in the form of raw or roasted kernels [5,6] are increasingly recommended for inclusion in healthy diets due to evidence of beneficial effects on blood cholesterol, coronary heart disease, diabetes, sudden death, inflammation, thrombosis and vascular reactivity [7–10]. More than 80% of the world trade in tree nuts is comprised of just four species, walnuts, hazelnuts, pistachios, and almonds [4]. Horticulturae 2017, 3, 24; doi:10.3390/horticulturae3010024 www.mdpi.com/journal/horticulturae Horticulturae 2017, 3, 24 2 of 10 Many other species of edible nuts around the world have potential to be developed and produce a commercial return, for example, macadamia and Brazil nuts are case studies. Macadamia originated in the sub-tropical rainforest of eastern Australia, and is a rare example of a nut from the tropics grown commercially in plantations [11–17]. Brazil nuts have also become an important industry for countries along the Amazon River such as Brazil, Peru and Colombia, however, Brazil nuts are harvested from the forest floor rather than plantations [18]. In Papua New Guinea, the Solomon Islands and Vanuatu, potential exists for domesticating a variety of new tree nuts that have been used in traditional cultures [19–21]. The genus Canarium contains tree nuts with beneficial properties, including Canarium odontophyllum, C. ovatum and C. album [22–26]. Canarium ovatum (Pili) is cultivated commercially in the Philippines, producing 23,000 metric tons of edible nuts in 2011 [19,27]. Canarium indicum L. (Burseraceae) is a tropical nut species indigenous to lowland rainforests of eastern Indonesia, Papua New Guinea (PNG), the Solomon Islands and Vanuatu, producing nutritious nuts and high quality timber [20,28]. Canarium nuts have been an important food for the people of Papua New Guinea for thousands of years [29,30]. Canarium indicum has great potential for commercialization and is in a unique position to develop as a sustainable industry, adding to food security in Melanesia [20,31,32]. Traditionally, the nut has been mainly harvested from the ground in forests, similar to the Brazil nut, but large numbers of trees are now being planted as shade trees in cocoa plantations in Papua New Guinea [20]. In traditional culture, surplus nuts are typically stored in woven baskets above a fireplace. This practice is not suitable for a commercial product. In other crops postharvest handling methods have been developed to reduce waste in the postharvest chain and maintain quality and nutritional attributes, [12–17,33]. Nuts are prone to rancidity because of high content of unsaturated fatty acids. Rancidity is one of the most important defects of this food [34,35]. Canarium nut has very high oil content of around 67 to 75%, with 40% oleic acid (monounsaturated, with one double bond) and 14% linoleic acid (polyunsaturated, with two double bonds) [36]. The high degree of unsaturation of canarium oil, especially linoleic acid, creates potential for oxidative rancidity to develop [37,38]. Hydrolytic rancidity can also occur if moisture content of foods is not reduced and maintained sufficiently [39,40]. It is the first step in the deteriorative rancidity pathway [39]. Fatty acids generated from hydrolysis are more prone to oxidation than intact triglycerides [38,41] and are the preferred substrate for respiration, generating heat and more water [42]. Tropical conditions with high temperatures and humidity may favour development of rancidity [43,44]. There is some knowledge of culture and postharvest processing of canarium nuts [45–49], however more research is needed to identify suitable postharvest treatments. We conducted trials to investigate postharvest shelf life of Canarium indicum nuts stored as whole nuts and as kernels. Canarium kernels were packed using a domestic vacuum packaging system and stored under ambient and refrigerated conditions to investigate the shelf life characteristics of canarium under conditions likely to prevail in the postharvest chain in a tropical climate. For comparison, kernels were also stored in a laboratory under ambient laboratory conditions that may approximate conditions sometimes pertaining within a building in the tropics. Knowledge of shelf life of canarium kernels will provide information on suitable storage conditions to maintain quality of nuts in conditions encountered in developing countries. 2. Materials and Methods 2.1. General Methods Canarium indicum fruit of up to two weeks post-abscission age was gathered from the ground in plantations in East New Britain, PNG. Fruit was depulped in PNG by a traditional method of soaking fruit in water, then allowing it to ferment in wet hessian sacks for two to three days until the pulp (exocarp) was soft enough to remove from the nuts. Sacks were maintained wet by external application of water as required. Three shelf-life experiments were conducted: (1) shelf-life of kernels Horticulturae 2017, 3, 24 3 of 10 stored in-shell as whole nuts at three different initial moisture contents (16.1% ± 0.3%, 10.3% ± 0.2% and 5.4% ± 0.1%) at mean laboratory temperatures of 22.2 ± 0.04 ◦C minimum and 24.5 ± 0.04 ◦C maximum; (2) shelf-life of kernels dried to an initial moisture content of 5.4% ± 0.1 stored at both ambient tropical temperatures (mean minimim 22.0 ± 0.01 ◦C to mean maximum 31.2 ± 0.26 ◦C) and under refrigeration (4 ◦C); and (3) shelf-life of kernels dried to an initial moisture content of 5.4% ± 0.1% stored at both laboratory temperature as above and under refrigeration (4 ◦C). Nuts were dried in a Memmert fan-forced laboratory oven (Memmert Gbh & Co. KG, Schwabach, Germany). The moisture content (wet basis (w.b.)) of nuts for shelf life studies was determined by drying three replicates of five nuts at 105 ◦C for 24 h to remove all free water [50] Moisture content was determined by the formula: Moisture content (w.b.) = (initial weight − final weight) × 100 / initial weight. All moisture contents were calculated on a wet basis (w.b.). For shelf-life of kernels, whole nuts were dried for 3 days at 38 ◦C followed by 3 days at 45 ◦C to achieve nut moisture content of 5.4% ± 0.1% (w.b.). Nuts were cracked by hand with a modified TJ’s™ macadamia nutcracker to release the kernels [45]. Kernels in testa were vacuum-packed for storage using a Foodsaver®Vac 440 domestic vacuum-sealer (Sunbeam Corporation, Botany, Australia) in polyethylene packaging film (Foodsaver VSO520, Sunbeam Corporation, Botany, Australia). At assessment kernels were blanched by pouring hot water at 100 ◦C over kernels in testa and allowing them to stand for 1.5 min before removing the testa. Kernels were then re-dried for 15 min at 60 ◦C to remove moisture absorbed during blanching.